Crankcase Ventilation

A mixture of crankcase air and oil mist is produced on each piston down stroke.
This forced mixture helps to splash lube the moving parts in the engine.
It also creates unwanted pressure in the engine.
The oil mist is then separated from the crankcase air and the excess air pressure is vented out of the engine.
86-90 engines vent through a valve in the cam cover.
91 and up engines vent through a valve in each rocker box and out the hollow breather bolts in each head.

1991-2003

In 1991, along with the 5-speed transmission, the MoCo moved the crankcase breathing from the cam box cover outlet to breather bolts in the heads.
Previously, these bolts were just used to mount the carb and air cleaner to the heads.

The new breather system uses one-way umbrella valves (AKA Foo Foo Valves) in the rocker boxes.
These exit crankcase vapors through vents in the top of the cylinder head into the carb mouth to be burnt. 10)

On each piston downstroke, crankcase pressure (air and oil mist) is routed up the pushrod tubes into the rocker box (bypassing the falling oil from the rocker boxes).

Collected air pressure and oil mist in each rocker box is routed up into a sealed cavity in the lower portion of the box.

This mixture passes up from underneath a rubber one-way valve (umbrella valve) sitting over the cavity inlet.

The oil is designed to separate from the air by hitting the underside of the umbrella valve and dropping back down into a recessed area behind the umbrella valve in the cavity.

From there it should drain back into the main rocker box through a tiny hole behind the umbrella valve and then back to the lower end.

The separated oil flows into drain holes in the rocker boxes and back into the gearcase.

Air pressure is designed to continue up past the umbrella valve and exit a hole in each head on the intake valve side.

Air pressure escapes the head through the hollow bolts (one in each head) that hold the air cleaner mount.

The Parts Catalog and FSM are a bit confusing regarding how many umbrellas are used and where.
The information is there but it's just not clear.
The parts catalog only shows an exploded diagram of one rocker box assembly on the Rocker Arms, Covers and Pushrods page.
Yet, the description shows that 2 umbrellas are required.
It's the same for all of the parts on that page as well as each newer parts catalog (1 pushrod, tube ,lifter and related parts).
This is the norm mostly throughout the subsequent catalogs with multiple (same) parts.
It's assumed this was done to save money on printing the catalogs (replicated information deleted).
The FSM displays the same exploded diagram as the parts catalog in the front of the Cylinder Head section.
It's only at the end of the Installation article does it say to install new gaskets on the middle rocker box (with the breather valve on the intake side).
There is no dedicated section on the rocker box or umbrella valves (which seem to be regarded as mere gaskets).

Installation

The middle rocker box is symmetrically cast so that it can be used on the front or rear head.
And, there is a spot (seemingly) for two umbrellas. 12)

However, only one umbrella is used in each middle section.
(only 2 umbrella valves are used in the entire engine.) 13)

Each umbrella is placed on the intake side on each head (only).

It is not always green as in the pic below (depends on what brand you buy).

91-03 crankcase ventilation is done through the hole with the wire tie. 17)

2004-Present

Just as with the 91-03 umbrella valves, only 1 breather valve per side is used.
And each one is placed on the intake side of each lower rocker box.

However, unlike the single umbrella valves in 03, each breather valve in 04 and up engines is directional.
(as in there is one for the front and a different one for the rear)

Each one consists of a poly fiber over a rubber flap. 18)
The fiber pieces are the same as the brown Scotchbrite pads that are sold in the body section of auto parts stores.
You can cut your own put them in the breather valves. They are aluminum oxide and won't hurt the engine. 19)

Each breather valve is assembled into a plastic fitting that is sealed over the outlet hole in the lower rocker box to the head breather bolt.